1. Field of the Invention
The present invention relates to an optical memory device, and more particularly, to an optical memory device operating according to a Time-Dependent Luminescence and Memory (TDLM) phenomenon which is actually a combination of two phenomena: a phenomenon that photoluminescence intensity is increased by exposure of the optical memory to excitation light; and another phenomenon that the photoluminescence intensity of the optical memory device before storage of the optical memory device is regained when the optical memory device is exposed to light after having been stored for a long period of time in a dark place without being exposed to light; namely, a retentive phenomenon.
Since the optical memory device can be subjected to rewriting and erasure a plurality of times, the optical memory device can be applied to fields such as information recording mediums, displays, image pick-up devices, image processing devices, retentive duplication, integration optical sensors, and multi-channel processors.
2. Description of the Related Art
Emission characteristics which do not change with time have been utilized for conventional light-emitting devices. The physical reason for such invariable characteristics is a very quick transition between energy levels.
The transition process is defined by Quantum mechanics and reflects interaction between carriers (electrons and holes) or excitons and photons. If the interaction (i.e., photo-(to-)electric or electric-(to-)photo conversion) is effected at very high speed within a considerably small space (on the order of atoms or molecules), luminous intensity also fluctuates at very high speed. Accordingly, in this case, the luminous intensity seems to remain unchanged on an ordinary time scale.
In most applications of light-emitting devices, from a practical viewpoint attention has conventionally been paid to materials other than nanoparticles(nano-size of particles); for example, a matrix (the continuous phase), such as polymers (Herron et al., and Buetje et al.), glass (Naoe et al.), and fluids, in which nanoparticles are embedded. Of these light-emitting devices, fluids are commonly used for measuring photoluminescence/spectrum of nanoparticles or visualization of color of the emitted light [see, for example, Dabbousi B. O., et al., J. Physic. Chem. 101, 9463 (1997)].
Light-emitting devices/mediums and optical processing devices/mediums, both using nanoparticles, are also disclosed. However, these employ light-emission characteristics which do not change with time. In all of these devices and mediums, nanoparticles (and their clusters) are spaced far apart from one another. Upon exposure to excitation light, each of the nanoparticles acts as a isolated single light-emitting substance. Such a structure of the light-emitting device is widely used for mediums such as light-emitting mediums or photoelectric materials (e.g., a photoelectric material disclosed by Herron et al.) used for producing an X-ray image.
High-density integration of nanoparticles, such as a nanoparticle film formed on a solid substrate or deposition of a nanoparticle layer, is of importance to application of nanoparticles to devices. A thin film of semiconductor nanoparticles is applied to a light-emitting diode (LED) (Alivisatos et al.), a photoelectric converting device [Greenham, N. C., et al., Phys. Rev. B, 54, 17628(1996)], an ultra high speed detector (Bhargave), an electroluminescence display and panel (Bhargave, Alivisatos et al.), a memory device of a nanostructure (Chen et al.) , and a multicolor device consisting of an arrangement of nanoparticles (Dushkin et al.) In most of these applications, nanoparticles are spaced in close proximity to one another within the thin film. Under certain conditions, nanoparticles exhibit a new photophysical property which is not observed in a single particle). The arrangement of particles [nanoparticle crystal (Murray et al.)] and the shift of the wavelength of emission (the red shift of the emission peak) of a patterned nanoparticle film (Dushkin et al.) are mentioned as examples of the photophysical property. The shift of the emission wavelength stems from long-distance resonance transportation of excitation among nanoparticles (kagan et al.).
However, for the functions of the devices and mediums the conventional art has not actively utilized the photophysical property that stems from the interaction among the particles. One conceivable reason for this is that the nanoparticle film does not have any definite (microscopic) structure and/or the structure is not uniform. Another conceivable reason is that basic interaction among the particles is cancelled by considerable interaction of electric fields (i.e., electroluminescence) (Alivisatos et al.).
The object of the present invention is to provide an optical memory device capable of increasing and storing luminous intensity.
To achieve the foregoing object, the present inventors actively utilized a collective function of a thin film for the first time, in which nanoparticles are arranged and integrated at high density. The collective function corresponds to the foregoing TDLM function of the nanoparticle film. By means of active utilization of such a function, formation of an image on the nanoparticle film can be achieved by utilization of an intensity ratio (contrast) between an exposed region and an unexposed region. The subject of the present invention is a unique photophysical property of a group of nanoparticles spatially arranged. This photophysical property differs from the physical properties of atoms and molecules, the physical properties of bulks, and the physical property of a single nanoparticle. The luminous intensity becomes greater by several orders of magnitude with time (typically with lapse of tens of minutes). At present, definite physical grounds for the process of shift in the arrangement of nanoparticles over a very long period of time still remain uncertain. Image pick-up and processing operations are examples of evident application of the foregoing phenomenon.
There will be given an explanation of a difference between the conventional technology and the TDLM phenomenon applied to the present invention.
All the conventional applications are directed toward generation of photons through recombination of carriers; namely, generation of carriers (electrons and holes) through interaction between photons and an external electric field, or vice versa. In contrast, the present invention employs the TDLM phenomenon, which is similar to optical pumping used for oscillating a laser (Sze, S. M., xe2x80x9cPhysics of Semiconductor Devices,xe2x80x9d Wiley, N.Y., 1981). The TDLM phenomenon corresponds to photo-to-photo conversion by means of generation and transportation of excitons or electron-hole pairs. Generation and transportation of electron-hole pairs is observed in a photo-refractive device/medium such as an electro-optical crystal (Valley et al.) or a polymer (Sutter et al.). A photo-refractive device/medium records an image on the basis of the principle that carriers (electrons and holes) are spatially separated by exposure of the photo-refractive device/medium to spatially-periodic photo and an external field, which involves a change in refraction factor. This principle is generally called dynamic holography (Peyghambarian et al., Nature, vol. 383, Oct. 10, 1996, pg. 481). Transportation of excitons or carriers in the TDLM phenomenon leads to emission of light of different wavelengths (i.e., colors) from an image and differs from the photo-refractive phenomenon.
A hole-burning effect (Naoe et al.) observed in a semiconductor nanocrystal differs from the TDLM phenomenon, as will be described below. The hole-burning effect is usually observed in a matrix (continuous phase); for example, glass, in which nanoparticles of a certain particle size distribution are dispersed in an isolated manner. Upon exposure to a monochrome laser beam (i.e., a laser having a single wavelength), only a group of nanoparticles of a specific particle size corresponding to the wavelength are excited, and a spectrum hole can be formed at the wavelength of the laser beam in the unevenly spread absorption spectrum. Memory devices utilizing the hole-burning effect have already been proposed.
The TDLM phenomenon differs from the hole burning effect in the following three points:
1) The hole-burning effect utilizes absorption, whereas the TDLM phenomenon utilizes emission.
2) In the case of the hole-burning effect, the total amount of excitons or carriers remains unchanged when pumping is effected through use of excitation light having a predetermined luminous flux. In contrast, in the case of the TDLM phenomenon, the total amount of recombination is increased.
3) In the case of the hole-burning effect, a spectrum hole usually has a very short relaxation time; i.e., several microseconds or less, at room temperature. In contrast, in the case of the TDLM phenomenon, the relaxation time is on the order of several hours at room temperature.
Another phenomenon observed in a single quantum dot is intermittent emission of light (fluorescence) [Nirmal et al., Nature, pg. 383, 802(1996)]. More specifically, when an isolated single CdSe nanoparticle is continuously exposed to a laser beam and is excited, the nanoparticle blinkingly emits light (at a characteristic time of about 0.5 second). The intensity of light emitted after the light has once been extinguished is constant and remains unchanged.
In contrast, the present invention has been conceived by focusing on a characteristic that nanoparticles can increase, or increase and memorize, a photoluminescence intensity as a function of time and by applying such a characteristic to a device.
Accordingly, the present invention purports to provide the following:
According to a first aspect of the present invention, there is provided an optical memory device comprising:
a luminous material capable of increasing and/or memorizing a photoluminescence intensity (hereinafter referred to as a xe2x80x9cluminous intensityxe2x80x9d) as a function of irradiation energy (time of excitation light or as a function of the dose) of excitation light.
According to a second aspect of the present invention, there is provided an optical memory device including luminous nanoparticles capable of increasing and/or memorizing a luminous intensity as a function of irradiation time of excitation light or as a function of the dose of excitation light, wherein
an organic compound including at least one device selected from the group comprising P, N, O and S exists in at least a portion of the surface of each luminous nanoparticle.
According to a third aspect of the present invention, there is provided an optical memory device comprising:
photoconductive material; and
luminous nanoparticles capable of increasing, or increasing and memorizing, a luminous intensity as a function of irradiation time of excitation light or as a function of the dose of excitation light, wherein the luminous nanoparticles and the photoconductive material are present in close proximity to each other.
According to a fourth aspect of the present invention, there is provided an optical memory device including luminous nanoparticles capable of increasing or increasing and memorizing a luminous intensity as a function of irradiation time of excitation light or as a function of the dose of excitation light, wherein at least a portion of the cluster of luminous nanoparticles comprises a protective layer formed from an insulating material.
According to a fifth aspect of the present invention, there is provided a nanoparticle thin film manufacturing method of forming on a solid substrate a thin film from extremely fine particles (hereinafter referred to as xe2x80x9cnanoparticlesxe2x80x9d) capable of increasing, or increasing and memorizing, a luminous intensity as a function of irradiation time of excitation light or as a function of the dose of excitation light, wherein
the solid substrate is coated, through spin coating with a suspension, which is formed by dissolving nanoparticles in a solvent in suspension, while being rotated at a speed of 500 rpm or more for ten seconds or more.
According to a sixth aspect of the present invention, there is provided a nanoparticle thin film manufacturing method of forming on a solid substrate a thin film from xe2x80x9cnanoparticlesxe2x80x9d capable of increasing, or increasing and memorizing, a luminous intensity as a function of irradiation time of excitation light or as a function of the dose of excitation light, wherein
the solid substrate is coated, through ink-jet coating, with a dispersed solution, which is formed by dispersing nanoparticles in an emulsion whose continuous phase is a water phase and whose dispersed phase is an oily phase.
According to a seventh aspect of the present invention, there is provided a photoluminescence erasing method, wherein after the luminous intensity of an optical memory devicexe2x80x94which includes luminous nanoparticles capable of increasing, or increasing and memorizing, a luminous intensity as a function of irradiation time of excitation light or as a function of the dose of excitation lightxe2x80x94has been increased, or increased and memorized, the optical memory device is subjected to heat treatment, thereby diminishing the luminous intensity.
According to an eighth aspect of the present invention, there is provided an integrating optical sensor comprising:
a thin film including luminous nanoparticles capable of increasing, or increasing and memorizing, a luminous intensity as a function of irradiation time of excitation light or as a function of the dose of excitation light; and
a mask including a region which is exposed to light to be measured and a region which is not exposed to the light.
According to a ninth aspect of the present invention, there is provided an optical disk comprising, as a recording medium, luminous nanoparticles capable of decreasing, or decreasing and memorizing, reflectivity as a function of irradiation time of excitation light or as a function of the dose of excitation light.
In the present invention, luminous nanoparticles are usually used in a state of a gathering of luminous nanoparticles. The gathering of luminous nanoparticles can be in a state of clusters of luminous nanoparticles, gathering of luminous nanoparticles and another particles, clusters of luminous nanoparticles and so on.